A crystal controlled frequency counter typically comprises a piezoelectric oscillator, a digital counter, an output display, and selected input, output and internal interface circuits. In these frequency counters, the piezoelectric oscillator provides the time base pulses to the digital counter to control the period during which the frequency of an applied signal is counted. The accuracy of the measured frequency result is dependent on the accuracy of the time base period which in turn is dependent on the accuracy of the piezoelectric oscillator.
Typically, the crystal resonators used in these piezoelectric oscillators are selected and processed by hand to obtain a resonator which produces a time base signal which is accurate to one part in 10.sup.6. These hand operations are labor intensive and include sorting, repeated lapping and polishing operations, and other fine detailing of the individual crystals to obtain the desired accuracy of the resultant time base signal.
To obtain a magnitude or better accuracy, many frequency controlled frequency counters include an oven in which the crystal is mounted. With such an oven, the ambient temperature of the crystal can be controlled within a narrow temperature range. By limiting the operational temperature range of the crystal, temperature induced frequency shifts can be greatly limited, and thus improve the frequency stability and, consequently, the accuracy of the time base signal. By using well known electronic tuning means in addition to the oven, the accuracy of the resonator frequency can be improved to one part in 10.sup.9 or slightly better.
It would be advantageous if an easily calibrated frequency counter using a batch processed crystal resonator were available. Such a batch processed crystal resonator can presently be manufactured having a frequency tolerance of one percent over a broad range of temperatures without the necessity of costly lapping, polishing and assorted other hand selection procedures of each crystal. It would also be advantageous if this frequency counter did not require the use of an energy intensive oven to maintain the crystal temperature within a closely held temperature range. The present invention represents such a frequency counter.